Focus-detector arrangements for generating projective or tomographic phase contrast recordings of a subject are widely known. For example, reference is made to the European patent application EP 1 447 046 A1 and the German patent applications (not yet published at the priority date of the present application) with the file references 10 2006 017 290.6, 10 2006 015 358.8, 10 2006 017 291.4, 10 2006 015 356.1 and 10 2006 015 355.3.
For imaging by ionizing rays, in particular X-rays, principally two effects can be observed which occur when the radiation passes through matter, namely absorption and the phase shift of the radiation passing through a subject. It is known that in many cases, the phase shift when a ray passes through a subject depends much more strongly on small differences with respect to the thickness and composition of the penetrated matter than the absorption does. In principle, the magnitudes of the two effects respectively depend on the energy of the radiation and the atomic number of the penetrated matter.
For such phase contrast radiography or phase contrast tomography, the phase shift of radiation due to the object is evaluated. Here, similarly as X-radiography or X-ray tomography, both projective images of the phase shift or even a multiplicity of projective images of tomographic representations of the phase shift can be calculated.
The phase of an X-ray wave cannot be determined directly, rather only by interference with a reference wave. The phase shifts relative to reference waves or neighboring rays can be measured by using interferometric gratings and compiled for projective and tomographic recordings. In respect of these interferometric measurement methods, reference is made to the documents cited above. In these methods, coherent X-radiation is passed through a subject, then delivered through a grating with a period adapted to the wavelengths of the radiation so as to create an interference pattern, which depends on the phase shift occurring in the object. This interference is measured by a downstream (subsequent=chronological) analysis-detector arrangement, so that the phase shift can be determined with position resolution. It is also known to produce such phase gratings for example by etching rectangular structures on a silicon wafer.
Previous systems for differential phase contrast radiography/tomography are configured for the parallel ray geometry. It has been found that the imaging in such systems functions satisfactorily only in the near-axial region, and becomes worse with an increasing fan and cone angle toward the edge of the detector.
For medical diagnosis and nondestructive material testing, a more compact structure of the X-ray system used is desirable. In computer tomography (CT) for example the source, the phase contrast grating system and the detector must rotate in a portal whose diameter is limited for system reasons and owing to the centrifugal force. The aperture of medical CT systems is furthermore dictated by the dimensions of the patient and the required ergonomics. These dimensions establish a maximal length for the beam path of the differential phase contrast imaging system. On the other hand, the field of view should be large enough to achieve a meaningful scan. This makes it necessary to use a wide fan or cone beam. Similar situations and considerations also apply to X-ray systems for projective recordings or C-arc systems.